This invention relates to nuclear reactors and, more particularly to a vacuum for a control rod guide tube of a boilingwater nuclear reactor. A major objective of the present invention is to provide for more thorough and convenient evacuation of particles and objects from the base of a control rod guide tube.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
To facilitate handling, fissile fuel is typically maintained in fuel elements. Typically, these fuel elements have a corrosionresistant cladding. The fuel elements can be grouped together at fixed distances from each other in a fuel bundle. A sufficient number of these fuel bundles are combined to form a reactor core capable of a self-sustaining chain reaction. Neutron-absorbing control rods are inserted into the core to control the reactivity of the core. The reactivity of the core can be adjusted by incremental insertions and withdrawals of the control rod.
Each control rod is housed within a vertical guide tube which ensures the vertical orientation and motion of the control rod. The control rod is moved using a control rod drive mechanism which is at least partially housed in a stub tube which also serves to support the base of the guide tube. The base of the guide tube has a diameter greater than the diameter of the stub tube so that the guide tube base can be a disk with a central hole through which the control rod drive uncoupling rod is mechanically coupled to the control rod.
Given that the control rods are the primary means for regulating reactor output, it is essential that the control rod drives remain fully functional. However, dust, sediment and various foreign objects falling into a drive mechanism housed in a stub tube can impair control rod drive functioning. Potentially, the control rod mechanism could jam, preventing control rod insertion and withdrawal. Alternatively, the foreign material can prevent the control rod drive mechanism from being removed for maintenance.
In the course of the present invention, foreign material, including rust particles, oxide dust, water impurities, and occasionally larger items, were found at the base of guide tubes. Turbulence, especially during removal and insertion of control rods could disturb these materials causing them to fall into the stub tubes, where they could impair control rod drive mechanisms. It was determined that sweeping was not an effective method of removing these materials from the guide tube base. Vacuums were problematic because they were difficult to position. Also, if the vacuum were great enough to remove the bulk of the material on the guide tube base, it could also remove control rod drive components from the stub tubes. For example, a control rod drive uncoupling rod could be pulled into the vacuum. What is needed is a system for cleaning control rod guide tubes which removes the bulk of material deposited on the base without disturbing the control rod drive mechanism.